Enthalpy‐enhancing gas is used to optimize the ionic conductivity of atmospheric plasma‐sprayed 3.9 mol% yttria‐stabilized zirconia (3.9YSZ) electrolyte. In the experiment, three hydrogen gas flow rates were used to
control the plasma energy. The size of the 3.9YSZ feedstock powder was sieved to be in the range of 20–45 μm. When the hydrogen gas flow rate increased, the sprayed surfaces became rougher. Nevertheless, the lowest apparent porosity,
the highest bulk density, and the hardest electrolyte were obtained at the hydrogen gas flow rate at 7 L/min. A 3.9YSZ electrolyte with an ionic conductivity of 1480 μ (Ω cm)‐1 was obtained at 800°C, when the
hydrogen gas flow rate was at 12 L/min. It was controlled by the grain‐boundary conductivity, which was closely related to the lowest migration energy, the highest density of the columnar grains, and the closest bonding between lamellar layers. The increase in the hydrogen gas
could diminish the amount of the migration energy and increase the grain‐boundary conductivity. On the other hand, a larger cubic lattice constant and a less amount of the monoclinic phase in the as‐sprayed electrolyte helped obtain greater intragrain conductivity.